Lorena B. Benseñor

IQGAP1 has been implicated as a regulator of cell motility because its overexpression or underexpression stimulates or inhibits cell migration, respectively, but the underlying mechanisms are not well understood. Here, we present evidence that IQGAP1 stimulates branched actin filament assembly, which provides the force for lamellipodial protrusion, and that this function of IQGAP1 is regulated by binding of type 2 fibroblast growth factor (FGF2) to a cognate receptor, FGFR1. Stimulation of serum-starved MDBK cells with FGF2 promoted IQGAP1-dependent lamellipodial protrusion and cell migration, and intracellular associations of IQGAP1 with FGFR1--and two other factors--the Arp2/3 complex and its activator N-WASP, that coordinately promote nucleation of branched actin filament networks. FGF2 also induced recruitment of IQGAP1, FGFR1, N-WASP and Arp2/3 complex to lamellipodia. N-WASP was also required for FGF2-stimulated migration of MDBK cells. In vitro, IQGAP1 bound directly to the cytoplasmic tail of FGFR1 and to N-WASP, and stimulated branched actin filament nucleation in the presence of N-WASP and the Arp2/3 complex. Based on these observations, we conclude that IQGAP1 links FGF2 signaling to Arp2/3 complex-dependent actin assembly by serving as a binding partner for FGFR1 and as an activator of N-WASP.

IQGAP1 is a homodimeric protein that reversibly associates with F-actin, calmodulin, activated Cdc42 and Rac1, CLIP-170, beta-catenin, and E-cadherin. Its F-actin binding site includes a calponin homology domain (CHD) located near the N-terminal of each subunit. Prior studies have implied that medium- to high-affinity F-actin binding (5-50 microM K(d)) requires multiple CHDs located either on an individual polypeptide or on distinct subunits of a multimeric protein. For IQGAP1, a series of six tandem IQGAP coiled-coil repeats (IRs) located past the C-terminal of the CHD of each subunit support protein dimerization and, by extension, the IRs or an undefined subset of them were thought to be essential for F-actin binding mediated by its CHDs. Here we describe efforts to determine the minimal region of IQGAP1 capable of binding F-actin. Several truncation mutants of IQGAP1, which contain progressive deletions of the IRs and CHD, were assayed for F-actin binding in vitro. Fragments that contain both the CHD and at least one IR could bind F-actin and, as expected, removal of all six IRs and the CHD abolished binding. Unexpectedly, a fragment called IQGAP1(2-210), which contains the CHD, but lacks IRs, could bind actin filaments. IQGAP1(2-210) was found to be monomeric, to bind F-actin with a K(d) of approximately 47 microM, to saturate F-actin at a molar ratio of one IQGAP1(2-210) per actin monomer, and to co-localize with cortical actin filaments when expressed by transfection in cultured cells. These collective results identify the first known example of high-affinity actin filament binding mediated by a single CHD.

The neurofibromatosis type 2 (NF2) tumor-suppressor protein Merlin is a member of the ERM family of proteins that links the cytoskeleton to the plasma membrane. In humans, mutations in the NF2 gene cause neurofibromatosis type-2 (NF2), a cancer syndrome characterized by the development of tumors of the nervous system. Previous reports have suggested that the subcellular distribution of Merlin is critical to its function, and that several NF2 mutants that lack tumor-suppressor activity present improper localization. Here we used a Drosophila cell culture model to study the distribution and mechanism of intracellular transport of Merlin and its mutants. We found that Drosophila Merlin formed cytoplasmic particles that move bidirectionally along microtubules. A single NF2-causing amino acid substitution in the FERM domain dramatically inhibited Merlin particle movement. Surprisingly, the presence of this immotile Merlin mutant also inhibited trafficking of the WT protein. Analysis of the movement of WT protein using RNAi and pull-downs showed that Merlin particles are associated with and moved by microtubule motors (kinesin-1 and cytoplasmic dynein), and that binding of motors and movement is regulated by Merlin phosphorylation. Inhibition of Merlin transport by expression of the dominant-negative mutant or depletion of kinesin-1 results in increased nuclear accumulation of the transcriptional coactivator Yorkie. These results demonstrate the requirement of microtubule-dependent transport for Merlin function.